Thyroid echogenicity: A clue to precise individual dosimetry in radioiodine therapy of hyperthyroidism

Abstract

Radioiodine therapy is a frequent option in treatment of patients with hyperthyroidism. Despite efforts to plan the thyroid absorbed dose by accounting for the gland size and radioiodine kinetics, the success of radioiodine therapy remains largely unpredictable. The current methods plan the mean thyroid radiation absorbed dose, assuming that it applies to target tissue - the thyroid follicular cells. However, the unique thyroid follicular structure and iodine kinetics may violate this assumption. Upon oral administration and capture by thyroid, the vast majority of time radioiodine spends in organified form in follicular colloidal lumen: the greater the follicle the more radiation is wasted before reaching the target cells. The (131)I radiation absorbed dose to thyroid follicular cells is less than the average thyroid dose, the more the greater the follicles. Thyroid echogenicity can be used to assess the amount of colloid in thyroid tissue, which in turn can be used to assess the follicle size and adjust the planned absorbed dose to patient-specific thyroid micro-architecture. Animal data on intrathyroidal iodine kinetics were considered in conjunction with model predictions that relate the size of thyroid follicles with (131)I irradiation of follicular cells. It turned out that the correction factors in the range 5-40% should be applied to oral activities of radioiodine calculated by the standard method. Next, several histology studies documented that normoechogenic thyroids have relatively large follicles, while hypoechogenic thyroids are mostly cellular, with almost empty, small follicles. All these concur with clinical data that Graves' disease patients with normoechogenic thyroids that received 200Gy in thyroid had comparable outcome to Graves' disease patients with hypoechogenic thyroid that received 100-120Gy in thyroid. Thyroid echogenicity is a probable clue to a better patient-specific dosimetry in radioiodine therapy of hyperthyroidism; direct evidences and precise estimates of benefits over current practices would be provided by controlled clinical trials.